The present invention is related to a method and an apparatus for the measurement of humidity and/or density on a measurement target using the microwave technique. The present invention is particularly related to non-contact measurement of a humidity value that is independent of the density and the thickness of the measurement target, and/or or a humidity-independent value of the density by surface, which can be converted into a density when the thickness is known. On dielectric materials, the measurement takes place by the transmission of electromagnetic radiation through the measurement target. The present invention can be applied to all kinds of measurement targets, in particular in the humidity measurement of big bales and cardboard boxes, filled for instance with tea, tobacco, cotton, textiles, pulps, organic materials and the like, and also for board-shaped measurement targets like for instance wood boards, press boards, MDF boards, building materials, gypsum, polystyrene foam, plastic materials and the like. The present invention is further suited for use with stationary or moving goods, like for instance grain, cereals, dried fruits and vegetables, granulates and the like.
From DE 40 04 119 C2, it is for instance known that humidity measurement with electromagnetic waves is possible on dielectric materials. Such a measurement is based on the fact that water has a great complex refraction coefficient n-ik, so that the dielectric properties of the humid measurement target are decisively determined by the dielectric properties of the water contained therein. Besides to the fact that water has a great complex refraction coefficient, it is important for the analysis of the measurement that the complex refraction coefficient is proportional to the density of the measurement target. Electromagnetic waves that interact with the measurement target experience attenuation in the measurement target, as well as a wavelength shortening. The attenuation of the electromagnetic waves is the greater the higher the material's reactive component k of the complex refraction coefficient is. The wavelength shortening of the electromagnetic wave is the more marked the higher the material's active component n of the complex refraction coefficient is. Therefore, both effects are the higher the greater the humidity and density of the material are.
The measurement of the complex refraction index of a measurement target is particularly successful with microwave resonators, compare DE 40 04 119 C2 e.g. Here, the change of the resonance width and the shift of the resonance frequency of the filled resonator with respect to the unfilled resonator are acquired, and by doing so, characteristic variables are determined for the reactive component and the active component of the complex refraction index or the complex dielectric constant, respectively, of the measurement target. Then, using further specific parameters, a density-independent humidity value and/or a humidity-independent density value are determined for the measurement target. Because the electromagnetic field is bound to the resonator and penetrates into the measurement target only up to a small depth of about half a wavelength, at spacious measurement targets and in particular when leakage sensors are used, resonator methods are in principle subject to the technical limitation to be able to capture humidity and density only at the surface, but not in the core of the measurement target. Moreover, the resonator must always be brought into direct proximity of the measurement target, mostly even be in contact with the same.
The detection of humidity and/or density in the core of spacious measurement targets, like for instance in big cardboard boxes or bales, is necessary in many applications. Transmission measurements are used in microwave technique for this, wherein microwave radiation is radiated through the measurement target.
From U.S. Pat. No. 4,727,311, a transmission measurement method is known in which the humidity content in the measurement target is determined using microwaves. For this purpose, two microwave input signals having different frequencies are used, and their respective attenuations are measured. The humidity is determined from the attenuation. In order to determine the density of the measurement target, the phase shift of the microwave signals in the measurement target is acquired.
From U.S. Pat. No. 5,333,493, an apparatus and a method is known for determining the humidity content in a sample which is transported through a measurement arrangement on a conveyor belt. A series of microwave signals having discrete frequencies is used for measurement. The phase shift of the received signals is averaged, and is analysed together with a weight of the measurement target determined by weighing, in order to determine the humidity.
From U.S. Pat. No. 6,025,724, an apparatus and a method is known for determining the humidity content in packaged goods. The phase shift and the attenuation of the received microwaves are measured plural times in this, the microwaves permeating the measurement target in different regions. Adding a further characteristic variable for the measurement target, a rough value for a corrected attenuation is determined from the measured values for the attenuation, which then serves to determine the humidity.
From US 2004/0239338 A1, a method and an apparatus for measuring the humidity and/or the salt content in a measurement target are known. Polarized microwave radiation is radiated through the measurement target in this, and the reflected radiation is analysed. The humidity is then determined from the attenuation of the reflected radiation.
From US 2005/0253595 A1, an apparatus and a method is known for determining the humidity, wherein the phase shift and the attenuation of the transmitted radiation are analysed. Further, the thickness of the sample is measured separately in order to analyse the humidity of the sample.
From an article by O. Schimmer et al., “Noncontacting determination of moisture content in bulk materials using sub-nanosecond UWB-pulses”, Microwave Theory and Techniques, IEEE Transactions, June 2005, a method is known in which a short quasi-Gaussian pulse of electromagnetic waves is generated and emitted via a sending antenna, which then penetrates through the measurement target and is received again by a receiving antenna. With the aid of multivariate calibration, of principal component regression (PCR) or of an artificial neuronal network, the shape of the transmitted pulse is compared with the shape of the transmitted pulse without measurement target. With this technique, at least two measurement variables can be obtained which can be used for density-independent humidity measurement.
From U.S. Pat. No. 4,319,185, a method is known in which an electromagnetic pulse is emitted whose height after transmission through the measurement target is used as a characteristic variable for the humidity. In this method, solely the attenuation is acquired, so that an additional density measurement is necessary to determine a density-independent humidity value.